Copper nanostructures and methods for their preparation

ABSTRACT

Copper nanostructures with relatively small dimensions and method for producing such structures are discloses. The ratios of the various reaction products may be adjusted to produce pentagonal nanowires and other structures such as tadpole shaped nanowires, nanocubes or pentagonal bi-pyramids.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 61/530,734, filed Sep. 2, 2011, which is incorporatedherein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT

The claimed subject matter was developed with Government support underNSF Grant Nos. 0804088, 1104614 and ECS-0335765, awarded by the NationalScience Foundation. The Government has certain rights in the claimedsubject matter.

BACKGROUND

Copper nanostructures have increasingly been found to have significantutility in the microelectronics and catalysis fields. For example,copper nanowires (e.g., polycrystalline wires that are usuallyfabricated by lithographic techniques) are currently used asinterconnects in computer chips. Copper nanostructures hold greatpromise for use in microelectronics including low-cost flexibledisplays, light-emitting diodes and thin film solar cells. Coppernanostructures have also been found to exhibit localized surface plasmonresonance (LSPR) peaks in the visible region. Copper nanoparticles havebeen widely used as catalysts for water-gas shift and gas detoxificationreactions.

Metal nanostructures in the shape of nanowires are believed to findwidespread use in applications such as the fabrication of transparentelectrodes for flexible electronic and display devices. They are alsouseful in formulating conductive coatings for electrostatic dischargingand electromagnetic shielding. Research has conventionally focused onuse of silver nanowires for use in such applications. Compared tosilver, copper is several orders of magnitude more abundant and issignificantly less expensive. Copper nanowires with reduced sizes (i.e.,reduced diameters) exhibit increased transmittance of visible lightmaking them even more ideal for electronics use.

A continuing need exists for copper nanostructures that are suitable foruse in various applications such as microelectronics and catalysis andfor methods for producing them. A particular need exists for coppernanowires with relatively small diameters and methods for producing suchnanowires.

SUMMARY

One aspect of the present disclosure is directed to a method forproducing a copper nanostructure. A reaction mixture is formed in areaction vessel. The reaction mixture includes a copper-containingcompound, a capping agent and a reducing agent. The copper-containingcompound is reduced with the reducing agent to cause copper to form acopper nanostructure. The pressure in the reaction vessel is less thanabout 190 kPa and/or the temperature of the reaction mixture is lessthan about 115° C. during formation of the nanostructure.

A further aspect of the present disclosure is directed to a populationof copper nanowire structures. Each structure has a length and adiameter. The average diameter of the copper nanowire structures is lessthan about 40 nm and the average ratio of length to diameter of thecopper nanowire structures is at least about 10:1.

Another aspect of the present disclosure is directed to a coppernanowire structure. The structure includes at least about 60 wt % copperand is characterized by a penta-twinned shape.

Various refinements exist of the features noted in relation to theabove-mentioned aspects of the present disclosure. Further features mayalso be incorporated in the above-mentioned aspects of the presentdisclosure as well. These refinements and additional features may existindividually or in any combination. For instance, various featuresdiscussed below in relation to any of the illustrated embodiments of thepresent disclosure may be incorporated into any of the above-describedaspects of the present disclosure, alone or in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an XRD pattern of a copper nanowire produced according toExample 1;

FIGS. 2-3 are SEM images of copper nanowire structures producedaccording to Example 1;

FIG. 4 is a TEM image of copper nanowire structures produced accordingto Example 1;

FIG. 5 is a graph showing the distribution of diameters of coppernanowires produced according to Example 1;

FIG. 6 is a TEM image of a portion of a copper nanowire producedaccording to Example 1;

FIG. 7 is a high-resolution TEM image of the region marked by the box inFIG. 6;

FIG. 8 is a TEM image of a second portion of a copper nanowire producedaccording to Example 1;

FIG. 9 is a high-resolution TEM image of the region marked by the box inFIG. 8;

FIG. 10 is a UV-vis spectra of an aqueous suspension of copper nanowireshaving an average diameter of about 24 nm and of silver nanowires havingan average diameter of about 80 nm;

FIG. 11 is a SEM image of copper bi-pyramids that formed after 30minutes of reaction as produced according to Example 3 with an insetshowing the SEM image of a tilted sample showing the pentagonalcross-section of the nanocrystals;

FIG. 12 is a SEM image of copper bi-pyramids that formed after 1 hour ofreaction as produced according to Example 3;

FIG. 13 is a SEM image of copper bi-pyramids that formed after 3 hoursof reaction as produced according to Example 3;

FIG. 14 is a SEM image of copper bi-pyramids that formed after 6 hoursof reaction as produced according to Example 3 with an inset showing theSEM image of a tilted sample showing the pentagonal cross-section of thenanocrystals;

FIG. 15 is a TEM image of the copper nanowire of FIG. 14;

FIG. 16 is a high-resolution TEM image of the region marked by the boxin FIG. 15;

FIG. 17 is a UV-vis spectra of the aqueous suspension of coppernanostructures of FIG. 11;

FIG. 18 is a SEM image showing one type of pentagonal bi-pyramid;

FIG. 19 is a geometric model of the bi-pyramid of FIG. 18;

FIG. 20 is a SEM image showing a second type of pentagonal bi-pyramid;

FIG. 21 is a geometric model of the bi-pyramid of FIG. 20;

FIG. 22 is a SEM image showing a third type of pentagonal bi-pyramid;

FIG. 23 is a geometric model of the bi-pyramid of FIG. 22;

FIG. 24 is a SEM image of copper nanocubes that formed after 30 minutesof reaction as produced according to Example 4;

FIG. 25 is a SEM image of copper nanocubes that formed after 1 hour ofreaction as produced according to Example 4;

FIG. 26 is a SEM image of copper nanocubes that formed after 6 hours ofreaction as produced according to Example 4;

FIG. 27 is a XRD pattern of the copper nanocubes produced according toExample 4;

FIG. 28 is a TEM image of a copper nanocube produced according toExample 4;

FIG. 29 is high-resolution TEM image of the region marked by the box inFIG. 28;

FIG. 30 is the UV-vis spectra of three separate aqueous suspensions of50 nm, 100 nm and 200 nm copper nanocubes; and

FIG. 31 is a schematic of the reaction pathways used to produce variouscopper nanostructures according to Examples 1-4.

DETAILED DESCRIPTION

The field of the disclosure relates to copper nanostructures and, moreparticularly, to copper nanostructures with relatively small dimensionsand methods for producing such structures. The ratios of the variousreaction products may be adjusted to produce other structures such astad-pole shaped nanowires, nanocubes or pentagonal bi-pyramids.

Provisions of the present disclosure are directed to coppernanostructures (e.g., nanowires) and methods for producing coppernanostructures. Without being held to any particular theory, it has beenfound that copper nanostructures formed at relatively low pressures(e.g., atmospheric pressure) and/or low temperatures (e.g., 100° C. orless) have a relatively small diameter. Further it has been found thatby adjusting the concentration of the components of the reaction mixtureand/or adjusting the respective ratios of the components, the shape ofthe resulting nanostructure may be changed.

Methods for Producing Copper Nanostructures

Generally the copper nanostructures of the present disclosure areproduced by forming a reaction mixture that contains a copper-containingcompound, a capping agent and a reducing agent. The copper-containingcompound is reduced by the reducing agent to produce elemental copperthat forms the nanostructure. During reduction, the pressure and/ortemperature of the reaction vessel may be maintained relatively low(e.g., a pressure of less than about 190 kPa and/or a temperature ofless than about 115° C.) such that nanowires with a relatively smalldiameter may be produced.

Suitable copper-containing compounds that may be included in thereaction mixture include any compounds from which elemental copper)(Cu⁰is formed upon contact with a reducing agent or during electrolysis oran electroless deposition method, or upon decomposition. Exemplarycopper-containing compounds include copper (II) nitrate (Cu(NO₃)₂,anhydrous or hydrated), copper (II) sulfate (CuSO₄, anhydrous orhydrated), copper (II) chloride (CuCl₂, anhydrous or hydrated), copper(II) hydroxide (Cu(OH)₂, anhydrous or hydrated), copper (II) acetate(Cu(CH₃COO)₂, anhydrous or hydrated), and copper (II) trifluoroacetate(Cu(CF₃COO)₂, anhydrous or hydrated). Suitable copper-containingcompounds may also include various ligands and/or chelates that containcopper without limitation.

The reducing agent that is combined with the copper-containing compoundis any compound (or ligand or chelate) that reduces copper ions intoelemental copper to deposit as a nanostructure seed or as part of thegrowing copper nanostructure. Suitable reducing agents include glucose(a or (3 form) and ascorbic acid.

In addition to the copper-containing compound and the reducing agent, acapping agent is included in the reaction mixture. The capping agentstabilizes the resulting nanostructure (e.g., by changing the surfaceenergies of different facets) and prevents aggregation between thestructures. The capping agent becomes incorporated into the matrixduring formation of the copper nanostructure-based composites. Suitablecapping agents include alkylamines. Alkylamines have the generalstructure of Formula (I) shown below

wherein R₁ is an alkyl group (or substituted alkyl group) and R₂ and R₃are either hydrogen or an alkyl group (or substituted alkyl group). Insome embodiments, the alkyl group of R₁ has 25 carbon atoms or less. Oneparticularly preferred alkylamine is hexadecylamine (“HDA”). HDA hasbeen found to be an effective capping agent for copper and has a strongselectivity toward the {100} facets of the nanostructure. In someparticular embodiments, HDA is used as a capping agent and glucose isused as a reducing agent. In such embodiments, copper nanostructures maybe produced in relatively large quantities with high purity and gooduniformity. Other alkylamines of Formula (I) that may be used includeoctadecylamine and oleylamine.

Generally, the components that form the reaction mixture are dissolvedin water; however in some embodiments an organic solvent may be used oreven a two-solvent system may be used. The copper-containing compound,the reducing agent and capping agent may be added to any suitablereaction vessel in any manner suitable to those of skill in the art(e.g., as solids or in solution form and in any order of addition).Suitable vessels may be lab scale (e.g., reaction vials) or may becommercial-scale (e.g., steel vessels which may be polymer-lined).Preferably the reaction vessel is agitated during formation of thecopper nanostructures. The nanostructures may be produced batch-wise orin a continuous manner (e.g., a continuous-stirred tank reactor (CSTR)).

Upon formation of the reaction mixture, the reaction contents areheated. Generally, the reaction mixture is heated to a temperature lessthan about 115° C. In some embodiments, the reaction mixture is heatedto a temperature less than about 110° C. or less than about 105° C.Preferably, the reaction mixture is heated to a temperature of 100° C.or less to prevent the reaction mixture from boiling causing thepressure of the reaction contents to increase as in pressurized vesselsystems. It is preferred that the reaction mixture be maintained atabout ambient pressure (101 kPa) or less. However in some embodiments,the pressure is maintained to be below about 190 kPa, less than about150 kPa, less than about 125 kPa or less than about 105 kPa.

In this regard, it has been found that by utilizing a reducedtemperature (e.g., less than about 115° C. and preferably less thanabout 100° C.) and/or a reduced pressure (e.g., less than about 190 kPaand preferably 101 kPa or less) copper nanostructures and, inparticular, copper nanowires are produced with a relatively smalldiameter (e.g., less than about 40 nm, less than about 30 nm or evenless than about 25 nm). Without being bound to any particular theory, itis believed the reduced temperature and/or pressure influencesnucleation of the copper nanostructure. It is believed that the seedsthat are produced at such reduced temperatures and pressures have adecahedral shape which allows nanowires having a penta-twinned structureto be produced. Such penta-twinned copper structures have a relativelysmall diameter compared to conventional copper nanostructures.

Generally the reaction is substantially complete after 6 hours or less.Other reaction times may be used depending on the concentration ofcomponents added, the desired structure of the nanomaterial and thedesired conversion. Reaction times may be at least about 30 minutes, atleast about 1 hour, at least about 3 hours, at least about 5 hours, fromabout 30 minutes to about 6 hours or from about 30 minutes to about 3hours.

The nanostructure that forms as a result of the process of embodimentsof the present disclosure depends on the relative reaction rates and, inparticular, the amount of reducing agent and/or capping agent present inthe reaction mixture. At relatively low reaction rates, decahedral seedsare nucleated and form penta-twinned nanowires with relatively uniformdiameter due to anisotropic growth (FIGS. 2-4). At greater reactionrates, isotropic growth is promoted during early stage of growth. As thereaction continues, the reaction rate becomes smaller and the structurenarrows to form a pentagonal bi-pyramid (FIG. 11). As the reactionproceeds, an even smaller reaction rate results and the pentagonalbi-pyramid further grows into tadpole-shaped nanowires (FIGS. 12-15).

In some embodiments, the reaction conditions are controlled such thatsingle crystal seeds are nucleated rather than decahedral seeds. Thisallows nanocubes (FIGS. 24-26) to form rather than nanowires and/orbi-pyramids.

In this regard, copper nanowires have been found to be produced withoutformation of bi-pyramids (FIGS. 2-4) at relatively low concentrations ofreducing agent and relatively high concentrations of capping agent. Ifthe concentration of reducing agent is increased, pentagonal bi-pyramidsform and the bi-pyramids taper off to form nanowires as the reactionproceeds. In contrast, if the concentration of capping agent is lowered,nanocubes form. Without being bound to any particular theory, it isbelieved that nanocubes may form due to oxidative etching. The oxidativeetching causes single crystal seeds to form which results in growth ofnanocubes. Such oxidative etching is blocked by the capping andprotective effect of the capping agent (e.g., HDA) at higherconcentrations of capping agent allowing multiply twinned copper seedsto form.

The relative molar concentrations between copper, reducing agent andcapping agent that may result in formation of the various structures areshown in Table 1 below. Generally these ratios were used in Examples 1-4described below.

TABLE 1 Relative amounts of components used to grow various coppernanostructures. NANO-BI- NANOWIRES NANOCUBES PYRAMIDS Concentration(mol/l) Copper 0.012 0.012 0.012 Capping Agent 0.075 0.037 0.075Reducing Agent 0.028 0.028 0.055 Molar Ratios Capping Agent/ 6.1 3.0 6.1Copper Reducing Agent/ 2.3 2.3 4.5 Copper Capping Agent/ 2.7 1.3 1.3Reducing Agent

In this regard, the relative amounts of the components may be adjustedto produce the desired structure as appreciated by those of skill in theart.

Copper Nanowires

Copper nanowires produced in accordance with the present disclosure arecharacterized by a relatively small diameter and a high aspect ratio.Generally, the population of copper nanowire structures that areproduced according to embodiments of the present disclosure have anaverage diameter of less than about 40 nm. In some embodiments thepopulation has an average diameter of less than about 30 nm, less thanabout 25 nm, from about 10 nm to about 40 nm, from about 10 nm to about30 nm from about 15 nm to about 40 nm, from about 15 nm to about 30 nm,from about 20 nm to about 40 nm or from about 20 nm to about 30 nm. Theaverage length of the copper nanowire structures produced according toembodiments of the present disclosure may be at least about 10 nm, atleast about 100 nm or even at least about 1 mm. In some embodiments, theaverage aspect ratio (i.e., the average ratio of length to diameter ofthe copper nanowire structures) is at least about 10:1. In otherembodiments, the aspect ratio is at least about 50:1, at least about100:1, at least about 1000:1, at least about 10,000:1 or even at leastabout 25,000:1.

The population of nanowires contains copper and amounts of organicmaterial (e.g., the capping agent). In this regard, the amount of copperin the population of nanowires (and in each nanowire) by at least about60 wt % copper or, as in other embodiments, at least about 70 wt %copper, at least about 80 wt % copper, from about 60 wt % to about 99 wt% copper or from about 70 wt % to about 95 wt % copper.

In this regard, the properties applied above may be an average of thepopulation of copper nanowires that is produced or of individualnanowires. Populations of copper nanowires may include at least about100 copper nanowires, at least about 1000 copper nanowires, at leastabout 10,000 copper nanowires, at least about 1×10⁶ copper nanowires oreven at least about 1×10⁹ copper nanowires.

The copper nanowires of the present disclosure have been found to have apenta-twinned structure (i.e., five single crystallites bound together).It is believed the penta-twinned structure is bound by ten {111} facetsat the two ends and five {100} side faces. It should be noted that thecopper nanowires are not constructed on a template or membrane. Incontrast, metallic copper atoms themselves give the nanowire itsstructural characteristics.

Other Nanostructures

As discussed above, other structures may be produced by varying thereaction conditions. In some embodiments, a tadpole shaped nanostructuremay be produced in which a bi-pyramid structure tapers from a base ofabout 200 nm (FIG. 11). If the reaction is allowed to continue, thereaction slows and a nanowire with a radius less than about 40 nmextends from the point of the bi-pyramid (FIGS. 12-15). In someembodiments, the reaction conditions are controlled such that coppernanocubes are formed. In the initial stage of reaction (e.g., at about 1hour), the cube sides are about 50 nm in size (FIG. 25). If the reactionis allowed to continue (e.g., for about 6 hours) the edges of the cubegrow to about 200 nm in size (FIG. 26).

EXAMPLES

The reaction conditions were varied in Examples 1-4 to produce variousstructures as shown in FIG. 31. It should be noted that other reactionconditions (e.g., component concentrations) may be used to produce thedesired nanostructures and the recited conditions are exemplary andshould not be considered in a limiting sense.

Example 1 Production of Copper Nanowires and Images Collected from Same

To produce copper nanowires, CuCl_(2.2)H₂O (0.021 g), HDA (0.18 g) andglucose (0.05 g) were dissolved in water (10 ml) in a vial (22.2 ml,borosilicate glass vial, with a black phenolic molded screw cap andpolyvinyl-faced pulp liner, VWR International (Radnor, Pa.)) at roomtemperature. After the vial had been capped, the solution wasmagnetically stirred at room temperature overnight. The capped vial wasthen transferred into an oil bath and heated at 100° C. for 6 hoursunder magnetic stirring. As the reaction proceeded, the solution changedits color from blue to brown and finally red-brown. All the chemicalswere obtained from Sigma-Aldrich (St. Louis, Mo.) and used as received.

To prepare samples for electron microscopy characterizations, theas-prepared aqueous suspensions were directly dropped onto siliconsubstrates (for SEM) or carbon-coated copper grids (for TEM andhigh-resolution TEM) and then dried under the ambient conditions of achemical laboratory. The silicon substrates or copper grids were thenrinsed with hot water (about 60° C.) to remove the excess HDA andglucose, followed by another round of drying. The products could havealternatively been collected as powders by use of centrifugationprocesses.

Scanning electron microscope (SEM) images were captured of the coppernanowires dried on silicon substrates. All SEM images were captured witha field-emission microscope (Nova NanoSEM 230, FEI (Hilsboro, Oreg.))operated at 15 kV. All transmission electron microscope (TEM) imageswere conducted with a microscope (Tecnai G2 Spirit, FEI (Hilsboro,Oreg.)) operated at 120 kV. High-resolution TEM imaging was performedusing a microscope (2100F, JEOL (Tokyo, Japan)) operated at 200 kV.Powder x-ray diffraction (XRD) patterns were recorded using adiffractometer (DMAX/A, Rigaku (The Woodlands, Tex.)) operated at 35 kVand 35 mA. The concentrations of Cu (II)/Cu (I) left behind in thereaction solutions were determined using an inductively-coupled plasmamass spectrometer (ICP-MS, PerkinElmer (Waltham, Mass.)).

FIG. 1 shows the X-ray diffraction (XRD) pattern of a copper nanowire.The three peaks at 20=43.5, 50.7, and 74.4° correspond to diffractionsfrom {111}, {200}, and {220} planes, respectively, of face-centeredcubic copper (JCPDS #03-1018). No other phases such as Cu₂O and CuO weredetected. The concentrations of Cu²⁺/Cu⁺ ions left behind in thereaction solution was measured using inductively-coupled plasma massspectrometry (ICP-MS). It was determined that the precursor had beenconverted into atomic copper at a percentage of 93%.

The scanning electron microscopy (SEM) image shown in FIG. 2demonstrates that copper nanowires could be prepared in high purity,typically approaching 95%, without any post-synthesis separation. Only avery small amount of copper nanocubes was found to co-exist with thenanowires. In addition, the nanowires were found to be highly flexibleand some of them showed bending more than 360 degrees without beingbroken. Both the SEM image at a higher magnification (FIG. 3) and TEMimage (FIG. 4) reveal that the nanowires were uniform in diameter andtended to be aligned in parallel to form bundles during samplepreparation. The nanowires had an average diameter of 24±4 nm ascalculated from 100 nanowires randomly selected from a number of TEMimages (FIG. 5). The lengths of the copper nanowires varied in the rangeof several tens to hundreds of micrometers; some of them were as long asseveral millimeters. The band-like contrast (see the box in FIG. 4)observed on the TEM images can be attributed to strains caused bybending or twisting.

FIGS. 6-9 show transmission electron microscopy (TEM) images and thecorresponding high-resolution TEM images taken from the middle (FIGS. 6and 7) and end portions (FIGS. 8 and 9) of two different Cu nanowires,respectively. The insets in FIG. 7 and FIG. 9 schematically illustratethe orientations of the copper nanowires relative to the incidentelectron beam (indicated by arrows). The high-resolution TEM images(FIGS. 7 and 9) show the existence of {111} twin planes parallel to thelong axis of the copper nanowire. When the direction of the e-beam wasperpendicular to the bottom side of the pentagonal nanowire (FIG. 7),two sets of fringes with lattice spacing of 2.1 nm and 1.3 nm wereobserved, corresponding to the {111} and {220} planes of copper,respectively. FIG. 9 shows the high-resolution TEM image taken from acopper nanowire oriented with one of its side faces parallel to thee-beam. The fringes with lattice spacing of 2.1, 1.8, and 1.3 Å could beindexed to the {111}, {200}, and {220} planes of copper, respectively.Based on the analysis of both SEM and high-resolution TEM images, it isevident that the copper nanowires had a penta-twinned structure bound byten {111} facets at the two ends and five {100} side faces, which areconsistent with the results previously reported for other metals (e.g.,Ag, Au, and Pd).

Example 2 Comparison of the UV-Vis Transmission Spectra Between theCopper Nanowires of Example 1 and Silver Nanowires

UV-vis spectra were taken with a diode array spectrophotometer (Cary 50,Varian (Palo Alto, Calif.)). FIG. 10 shows UV-vis transmission spectrarecorded from aqueous suspensions of the 24-nm copper nanowires ofExample 1 and penta-twinned silver nanowires of 80-nm in diameter(prepared according to the literature) at roughly the same metalconcentration (30 μg/ml), suggesting a slightly higher transmittance inthe visible region for the copper nanowires. This higher transmittancecould be attributed to the smaller diameter of the copper nanowires.

Example 3 Production of Copper Nanostructures with a Bi-Pyramid Shapeand Images Collected from Same

The preparation procedure of Example 1 was used to produce coppernanocrystals but the concentration of glucose (i.e., the reducing agent)was increased from 5 to 10 mg/ml. As can be seen from FIGS. 11-16,tadpole-like copper nanostructures resulted from the increased amount ofreducing agent. In an effort to uncover the growth mechanism, theproducts obtained at different reaction times were analyzed as detailedin FIGS. 11-16. In the initial stage (t=30 min), the solution changedits color from blue to red-brown due to the formation of tapered coppernanocrystals whose diameter gradually changed from 200 to 25 nm over alength of 0.5 to 1 μm (FIG. 11). The tapered cooper nanocrystalsexhibited a UV-vis absorption peak around 591 nm (FIG. 17) and arecharacterized by a pentagonal bi-pyramid structure (see inset of FIG. 11and FIGS. 18-23) formed by stretching apart the five-fold apices of adecahedron.

After the reaction had proceeded to 1 hour (FIG. 12), thin coppernanowires of about 24 nm in diameter started to appear from the thinnerend of a tapered nanocrystal. As the reaction was continued for threehours (FIGS. 13 and 14), the copper nanowires further grew along thelong axes with almost no change to their diameters. These resultsindicate that the tadpole-like copper nanowires originated from thetapered nanocrystals. The SEM image in the inset of FIG. 14 indicatesthat the tadpole-like copper nanowires also had a pentagonalcross-section. The TEM and high-resolution TEM images shown in FIGS. 15and 16 further confirm a tadpole-like morphology and a penta-twinnedstructure for the copper nanowires.

Example 4 Production of Copper Nanostructures with a Cubic Shape andImages Collected from Same

The preparation procedure of Example 1 was used to produce coppernanocrystals but the concentration of HDA (i.e., the capping agent) wasdecreased from 18 mg/ml to 9 mg/ml. Copper nanocubes (FIGS. 24-26)formed rather than copper nanowires. FIGS. 24-26 are SEM images of theproducts obtained after 0.5 h, 1 h, and 6 h of reaction, respectively.FIG. 27 gives an XRD pattern of the nanocubes obtained at 6 h. For bulkcopper, the strongest XRD diffraction is the (111) peak, followed by the(200), (220), and (311) peaks. In contrast, the copper nanocubes tend togive (200) diffraction as the strongest peak because of theirpreferential orientation with {100} planes parallel to the substrate.The high-resolution TEM image of an individual Cu nanocube viewed alongthe <100> zone axis (FIGS. 28-29) clearly shows well-resolved,continuous fringes with lattice spacing of 1.8 Å, corresponding to the{100} planes, indicating that the nanocube was a single crystal bound by{100} facets.

FIG. 30 shows UV-vis absorption spectra taken from the copper nanocubesdispersed in water. The copper nanocubes exhibited a major SPR peak inthe visible region, whose position was red-shifted from 565 to 625 nm asthe edge length of the nanocubes was increased from 50 to 200 nm.Compared to silver nanocubes with a similar size, the SPR peak of thecopper nanocubes was positioned at a much longer wavelength.

When introducing elements of the present disclosure or the preferredembodiments(s) thereof, the articles “a”, “an”, “the” and “said” areintended to mean that there are one or more of the elements. The terms“comprising”, “including” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

As various changes could be made in the above apparatus and methodswithout departing from the scope of the disclosure, it is intended thatall matter contained in the above description and shown in theaccompanying figures shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:
 1. A method of producing a copper nanostructure, themethod comprising: forming a reaction mixture in a reaction vessel, thereaction mixture comprising a copper-containing compound, a cappingagent and a reducing agent; and reducing the copper-containing compoundwith the reducing agent to cause copper to form a copper nanostructure,wherein (1) the pressure in the reaction vessel is less than about 190kPa and/or (2) the temperature of the reaction mixture is less thanabout 115° C.
 2. The method as set forth in claim 1 wherein the pressurein the reaction vessel is about atmospheric pressure.
 3. The method asset forth in claim 1 wherein the capping agent is an alkylamine.
 4. Themethod as set forth in claim 3 wherein the alkylamine has less thanabout 25 carbon atoms.
 5. The method as set forth in claim 1 wherein thereaction mixture comprises water as a solvent.
 6. The method as setforth in claim 1 wherein the temperature of the reaction mixture duringformation of the nanostructure is less than about 115° C.
 7. The methodas set forth in claim 1 wherein the copper-containing compound isselected from the group consisting of copper (II) sulfate, copper (II)chloride, copper (II) hydroxide and copper (II) nitrate, copper (II)acetate and copper (II) trifluoroacetate.
 8. The method as set forth inclaim 1 wherein the reducing agent is selected from the group consistingof glucose and ascorbic acid.
 9. The method as set forth in claim 1wherein the capping agent is selected from the group consisting ofhexadecylamine, octadecylamine and oleylamine.
 10. The method as setforth in claim 1 wherein the capping agent is hexadecylamine.
 11. Themethod as set forth in claim 1 wherein the concentration of at least oneof the reducing agent and the capping agent in the reaction mixture iscontrolled to produce a copper nanostructure in the shape of (1) ananowire, (2) a pentagonal bi-pyramid, (3) a nanowire having a tad-poleshaped portion or (4) a nanocube.
 12. A population of copper nanowirestructures, each structure having a length and a diameter, the averagediameter of the copper nanowire structures being less than about 40 nmand the average ratio of length to diameter of the copper nanowirestructures being at least about 10:1.
 13. The population as set forth inclaim 12 wherein the average ratio of length to diameter of the coppernanowire structures is at least about 50:1.
 14. The population as setforth in claim 12 wherein the average diameter of the copper nanowirestructures is less than about 30 nm.
 15. The population as set forth inclaim 12 wherein the copper nanowire structures comprise at least about60 wt % copper.
 16. The population as set forth in claim 12 wherein thepopulation comprises at least about 100 copper nanowires.
 17. A coppernanowire structure, the structure comprising at least about 60 wt %copper and being characterized by a penta-twinned shape.
 18. The coppernanowire structure as set forth in claim 17 wherein the nanowirestructure comprises at least about 60 wt % copper.
 19. The coppernanowire structure as set forth in claim 17 wherein the ratio of thelength to the diameter of the copper nanowire structure is at leastabout 10:1.
 20. The copper nanowire structure as set forth in claim 17wherein the diameter of the structure is less than about 40 nm.